Many industrial operations such as machining of metals, drilling, grinding, turning, milling, rolling, wire drawing or swaying require so-called functional fluids.
The role of these fluids is to decrease cutting forces, to cool the workpiece to obtain good dimension characteristics, to remove the chips from the cutting zone, to impart a good surface funish to the workpiece and to extend the life of tools.
Water is the best cooling agent because its specific heat, heat of vaporization and thermal conductivity are high. At the same time, it is the most economical and most harmless cooling agent towards the environment. However, the use of aqueous functional fluids is becoming common. These aqueous functional fluids may be true aqueous solutions of different additives in water which are called synthetic fluids, or alternatively, microemulsions which are called semisynthetic fluids. Semisynthetic fluids contain, in addition to water, mineral oils and surfactants.
For high pressure machining operations, the use of aqueous functional fluids has not yet found a satisfactory solution.
In these operations, the friction between metal surfaces becomes very high; and it is necessary to use extreme pressure additives. The role of these additives consists in forming a protective layer on the metal surfaces. This protective layer film prevents a work-piece from being seized, or even worse, becoming welded to the working tool.
The most commonly used among these extreme pressure additives are sulphur-containing additives because they are the most effective. Contact with hot metal surfaces gives rise to the decomposition of sulphur-containing products and to the formation of a protective metal sulphide layer which is continually renewed.
The sulphur-containing products used as extreme pressure additives in lubricants based on mineral oils or emulsions are dialkyl polysulphides, sulphur-containing polyisobutenes and sulphur-containing fatty acid esters. All these products are sparingly soluble or insoluble in water.
Attempts were made to overcome this disadvantage by using extreme pressure additives which are soluble in water.
The article by R.W. MOULD of British Petroleum Co. Ltd. (Journal of the American Society of Lubrication Engineers, 33(6)291-298 (1977)) examines the efficiency of a number of water-soluble halogenated or sulphur-containing products as extreme pressure additives for aqueous fluids. The efficacy of halogenated, generally chlorinated, products is very low. The sulphur-containing products studied, such as sodium salts of thiosalicylic acid, or 2-mercaptopropionic acid, 2,2'-dithiodibenzoic acid, 2,2'-dithiodipropionic acid, disodium-L-cystine and disodium dithiodiglycolate are not very stable and promote development of bacteria and release of hydrogen sulphide.
To stabilize them, the formulations of these products require addition of large quantities of bactericidal agents which are commonly used in emulsions, but are generally excluded from functional fluids.
U.S. Pat. No. 4,250,046 describes the use of diethanol disulphide as extreme pressure additive. However, this product is incompatible with many additives which are commonly used in the formulation of semisynthetic fluids.
We have now found a water-soluble, effective and stable extreme pressure additive.